This invention relates to the art of geophysical prospecting in general and to the art of radiactivity well logging in particular and more particularly to improvements in such logging wherein the lifetime of neutrons in the formation is measured.
Radioactivity well logging is conventionally of three sorts, natural gamma ray logging, gamma ray-gamma ray logging, and neutron logging. Neutron well logging is sometimes performed by traversing a well with a radium-beryllium source of energetic neutrons, thereby continuously irradiating the formations penetrated by the well with fast neutrons, and simultaneously measuring and recording the number of neutrons or gamma rays returning to the well from the formations as a result of such irradiation.
An improvement in neutron well logging as shown in the prior art utilizes a periodically varying neutron source whereby the neutron irradiation of the formations is periodically cut off. Such a source may comprise a system employing a material emitting alpha particles with a disk rotating beside it having alternate sectors made of material which emits neutrons when exposed to alpha particles. The disk is rotated to turn the neutron source off and on at an audio frequency. A continuously operating detector detects secondary radiation emitted from the formations as a result of the radiations, and the audio-frequency component in the detection signal is amplified and recorded to the exclusion of any constant detection signal related to the natural gamma radiation. In the instant invention, not only the neutron source but also the detecting system is periodically varied.
It is known in the prior art to irradiate the formations for a period of time sufficient to activate an appreciable number of nuclei of material in the formations, thereby rendering the material artificially radioactive, and at an interval of time following the cessation of irradiation to measure this artificial radioactivity. The time is related to the half life of the activated material and is of the order of minutes or seconds for such of the common elements of the earth as are activated appreciably by this means. This may be accomplished by causing the source to be turned on and off at a frequency related to half life and detecting continuously or the detector may follow a continuously emitting neutron source through the well at such spacing and rate as to cause the detector to arrive at a particular point in the well a predetermined interval after irradiation. In the instant invention, the period of the periodically varying source and detector system is related not to the half life of the irradiated material but to the lifetime of the neutrons in the formations, that is, to the time needed for fast neutrons emitted from the source to be slowed and captured. This time is of the order of hundreds of microseconds in ordinary earth formations.
In U.S. Pat. No. 3,379,882, assigned to the assignee of the present invention, there is described a system for radioactivity well logging wherein the formations are irradiated with neutrons from a periodically varying source operating at a repetition rate of the order of magnitude of hundreds or thousands of cycles per second, being thus alternately on and off for periods of hundreds of microseconds. A detecting system is synchronized with the source to operate while the source is off. The radiation detected may be either slow neutrons or gamma rays which result from thermal neutron capture; in either case the signal is related to the population of thermal neutrons in the formation, surrounding the borehole. From the time the source is turned off, the slow neutrons are gradually captured and the amount of radiation detected per unit time decreases until the source is again turned on. By measuring the number of thermal neutrons (or gamma rays produced by the thermal neutrons) present at any particular time, the rate of decay of the thermal neutron population may be measured. The rate of decay is dependent upon the nuclei of the material present in the formation and varies from formation to formation. This rate of decay is related to the time required for those nuclei to capture the thermal neutrons; therefore, in the system described therein, the measurement is related to this time and hence to the lifetime of the neutrons in the formations. The radioactivity well log which is normally conducted according to the radioactivity well logging system described in U.S. Pat. No. 3,379,882 comprises a pair of counting rate curves plus a curve indicative of the rate of decline of the neutron population which is a function of the ratio of the two counting rate curves. The curve is calibrated to record sigma, the thermal neutron capture cross section of the formation. The early gate, or N.sub.1 curve can be derived during some interval, for example, 400 to 600 microseconds, or 400 to 680 microseconds after time zero as contemplated by the present invention; that is, it is a measure of the radiation intensity detected during the interval from 400 to 680 microseconds after the termination of the neutron pulse from the neutron source. The late gate, or N.sub.2 curve, is a similar measurement derived during the interval from 700 to 980 microseconds after time zero. The present invention relates to an improvement upon the system as described in said patent.
Furthermore, in U.S. Pat. No. 3,706,884, also assigned to the assignee of the present invention, there is described a system for using three detection gates following each neutron pulse and associated circuits for substantially eliminating radiation background count in pulsed neutron well logging. Although the system described in U.S. Pat. No. 3,706,884 has worked with a high degree of success, there has sometimes been a problem when using the three gate system because of statistical inaccuracies arising due to the count rate of the signals being of the same order of magnitude.
Furthermore, in U.S. Pat. No. 3,566,116 to William B. Nelligan, especially with regard to FIG. 6 of that patent, means are provided for pulsing the neutron generator three times followed by the omission of a single neutron pulse in an attempt to measure the background radiation. However, this particular system, by skipping every fourth neutron pulse, does not recognize the problem associated with disabling the pulse generator so often with its concomitant loss of neutrons. Furthermore, with such a system, the pulse generator is not likely to be disabled for a period sufficient to allow all the background radiation to die away.
It is therefore the primary object of the present invention to provide an improved method and apparatus for determining geophysical characteristics of formations about a borehole;
It is still another object of the invention to provide means for computing and/or eliminating the effect on the counting rated due to background radiation within the borehole.
The objects of the invention are accomplished, generally, by means for pulsing a neutron generator for a considerable number of times and for allowing the generator to be off for a small percentage of time compared to the number of times the generator is pulsed in order to measure the background radiation while having no substantial effect on the number of neutrons generated.